Embodiments of the present disclosure generally relate to shock absorber assemblies that may be used with power drive units of vehicles, such as an aircraft.
Aircraft typically include movable control surfaces for directional control in flight. Known control surfaces include ailerons for roll control, elevators for pitch control, and rudders for yaw control. Additionally, various jet transport aircraft include leading edge slats and trailing edge flaps on the wings, which may be used to generate high lift during takeoff and landing when the aircraft is traveling at relatively low air speeds.
Power drive units (PDUs) are typically used to drive high lift surfaces on transport aircraft. Each surface is driven by two drive stations. A single drive line is routed from a PDU to actuators on both sides of the aircraft. In the event of a mechanical jam, or when the system is inadvertently driven into an over-travel stop, all the available PDU torque concentrates into the point of the jam. Torque limiters and torque brakes are often employed to limit the maximum amount of torque that may be delivered to specific points in the drive system.
In addition to local torque brakes at each actuator, half system torque brakes are sometimes used to limit the amount of torque delivered to one wing, thereby allowing a reduction in the size of drive line components between the PDU and the point of the jam. When an actual jam occurs at one or more points downstream of the half system torque brake, a second jam occurs in the drive line between the motor and the half system torque brake after the half system torque brake locks.
In general, a magnitude of the jam torque experienced by the drive system is equal to the stall torque of the primary mover (such as a hydraulic or electric motor) plus the torque supplied by the kinetic energy of the motor rotor. The kinetic torque is influenced by the driveline stiffness between the motor rotor and the point of the jam. With half system torque limiters, the distance between the motor and the torque brake is relatively short. Indeed, the two devices are typically mounted to the same housing. As such, the stiffness between the torque brake and the motor is relatively high, which leads to an induced kinetic torque to also be relatively high. Often, the kinetic torque approaches or exceeds the value of the motor stall torque. In order to reduce the magnitude of the kinetic torque, the motor rotor is decelerated over a period of time (generally, the longer the deceleration time, the less than magnitude of the kinetic torque). One method of decelerating the motor rotor is to introduce compliance in the drive line path between the motor and the torque brake.
Known half system torque brakes often utilize shock absorbers that include ring springs (also known as Fedder springs). With these shock absorbers, when torque in a first output shaft exceeds a predetermined level, a ball rolls up ramps machined into cam plates, thereby compressing Belleville springs and clamping brake plates. When the output shaft of the PDU stops rotating, substantial kinetic energy is still present in the motor rotor, which leads to additional kinetic torque that causes an input cam plate to continue to rotate with respect to an output cam and the ball to roll farther up the ramps. The continued motion of the ball causes the output cam to axially move, thereby compressing ring springs. In general, a shock absorber is operatively connected to each torque brake. Further, if the ring springs resonate, the cam plate may be susceptible to reversing direction and unlock the torque brake, thereby allowing excess torque to bleed through to the output shaft.
As can be appreciated, using a shock absorber with respect to each torque limiter adds costs to the overall system. Further, each shock absorber includes numerous parts, such as the individual ring springs, thereby adding weight and expense to the system.
Accordingly, a need exists for a more efficient system and method of absorbing shocks within a PDU.